The long-term goal is to understand the contractile mechanism, the basis of "resting" stretch resistance in smooth muscle, and the control of these functions by neurotransmitters, hormones, ions and pharmacological agents. Smooth muscle fibers in main pulmonary artery, and pulmonary arterioles of mammals, are being studied using electrophysiological, pharmacological, histochemical and ultrastructural methods. Details of the innervation of the fibers, whether by catecholaminergic or other nerves and the relationship of fibers to one another and to connective and elastic tissue are being investigated. Peptide-secreting cells are sought as well. The electrical parameters of the resting membrane and the ionic basis of the resting membrane potential are under investigation. Apparatus is being developed for study of mechanical responses to pharmacological agents in resistance vessels less than 250 microns in diameter.